1. Field of the Invention
This invention relates to a finder optical system which is suitable for a single reflex electronic camera electrically performing image processing by using an imaging unit, such as an image pickup tube or a CCD.
2. Description of the Prior Art
Heretofore, single reflex cameras using silver salt films, such as 35 mm films, are becoming the most suitable cameras for use in performing system development. A schematic diagram of a typical prior art finder optical system configuration of a single reflex camera is shown in FIG. 1. In the figure, there are shown a rotatable reflecting mirror 101, a shutter unit 102, a film surface 103, a focusing screen 104, a penta roof prism 105, an eyepiece 106 and a pupil 107 for observation. The finder optical system shown in FIG. 7 has an excellent optical performance of not less than 90% of the ratio of the picture photographed on the film surface to the image of the object observed in the finder optical system, i.e., the field of view ratio, and not less than 0.8 of the magnification of the field of view i.e., field of view magnification in the case of mounting a standard lens. In addition, this finder optical system has a feature that the total device can be constructed in a relatively small size.
However, when the aforementioned penta roof prism is used in the finder optical system of the recently introduced electronic cameras using an imaging unit such as a CCD, it becomes difficult to obtain the same degree of the field of view ratio and the same amount of field of view magnification, as in a conventional single reflex camera. It also becomes difficult to realize miniaturization of the total device. The reasons are as follows:
(1) For example, the effective picture of a 2/3 inch imaging unit is, in the ratio of diagonal length, as small as about 1/4 that of 35 mm film, so that if the conventional penta roof prism is used, the optical path length becomes too long, resulting in difficulty of obtaining a high field of view ratio and a high field of view magnification.
(2) Much space is required in the rear portion from the imaging unit for disposing an electric processing circuit, so that the distance from the image surface of the photographic lens to the utmost rear portion of the camera becomes long. For this reason, the position of the pupil of the finder optical system should be extended in the rear portion of the camera side, resulting in difficulty of obtaining a high field of view ratio and a high field of view magnification.
(3) The photographic lens is constituted to be telecentric for the color separation of the imaging unit, so that the spreading of the effective luminous flux from the photographic lens is large in a region in which the finder optical path is divided, resulting in a larger reflecting mirror.
(4) Much space is required in the front portion from the imaging unit for disposing optical units such as a low-pass filter, an infrared-cut filter and a protective glass, so that the distance between the dividing point of the finder optical path and the image surface should be large, resulting in a larger size of the total device.
Now, for reference purposes, FIG. 2 shows a schematic diagram of a finder optical system intended to achieve not less than 90% of the field of view ratio by using a conventional penta roof prism in an electronic camera.
In FIG. 2, there are shown a photographic lens 200, a dividing unit 201 for dividing the optical path from the photographic optical path into the finder optical system, a low-pass filter 202, a shutter unit 203 and an imaging surface 204 of an imaging unit 205. In the front portion of the imaging unit 205 there is disposed a protective glass having an infrared-cut effect. There is also shown a finder optical system unit 206 including an orthoscopic image erecting system, a focusing screen 207, an electric processing circuit unit 208 for imaging signals and a pupil for observation 209.
In general, as the magnification of the field of view .gamma. becomes larger, the observation of the finder image becomes easier. The field of view magnification may be represented as =f.sub..theta. /f.sub.e, where f.sub..theta. is the standard focal length of the photographic lens, and f.sub.e is the focal length of the eyepiece. In order to make the magnification of the field of view large, it is necessary to make the focal length of the eyepiece f.sub.e small, since the focal length of the standard lens f.sub..theta. is nearly constant. The eyepiece is disposed so that the front focus thereof is situated near the finder image surface of the finder optical system. Theefore, in order to make the magnification of the field of view large, it is necessary to make the optical passage length of the optical system for obtaining an orthoscopic erecting image from the focusing screen to the eyepiece as short as possible.
However, in the case of an electronic camera, as described above, it is necessary to dispose units such as an infrared-cut filter, a low-pass filter and further an electric processing circuit which electrically processes image information, in front of and behind the image surface of the photographic system. Hence it becomes necessary to shift the movable mirror (the quick-return mirror) guiding the optical flux of the object to the finder optical system to a position more toward the object side (in front), and the observing position more toward a rearward position.
Accordingly, when a construction is adopted wherein the finder image on the focusing screen of the photographic lens is directly observed with the eyepiece, the focal length of the eyepiece becomes necessarily long, resulting in a smaller magnification of field of view and a finder which is difficult to observe. It has been proposed, for example, to have a shorter focal length of the eyepiece, e.g. Japanese Patent Provisional Publication No. 60-43628 (1985); but even in this case the magnification of the finder is limited to about 0.5.
Furthe, the photosensor in the single reflex camera using 35 mm film is, in general, disposed near the eyepiece or near a side portion of the field lens. However, for both cases, the disposition of the photosensor requires a provision of a new space for disposing the photosensor, which is unsuitable for a compact camera. On the other hand, the photographic information display device is also disposed on the image surface of the objective, i.e., on a side portion of the field lens in the side of the camera body.
In recently introduced electronic cameras, however, a space occupied by an electric mounting, such as a processing unit which electrically processes the image, a shutter unit, and further flexible printed circuit boards for electrically connecting these units is large, so that providing a new space itself has become very difficult.